A variety of superemitting materials have been previously described in U.S. Pat. Nos. 4,776,895 and 4,793,799, both issued to Goldstein. Selective emission arises from such superemitting material due to the decay of inner electron shell transitions in a solid phase. Some superemitting materials often have an element present in a mixed oxidation or mixed valence state, forming a nonstoichiometric oxide. Some of the most effective superemitting materials in this class of compounds having mixed oxidation or mixed valance states include rare earth/alkaline earth oxide systems, rare earth/transition metal oxide systems, actinide oxides and the like, and various other mixed metal oxide systems.
Superemitters, when heated to a threshold temperature, emit visible or infrared radiation in a wavelength band that is related to the inner electron shell vacancy of the particular superemitter material. Radiation emitted from such superemitters is often in the form of a narrow band and can, therefore, be absorbed efficiently by a photovoltaic device, such as a silicon cell, InGaAs, and the like, to convert the radiation to an output voltage and current. The thermally-stimulated superemitters produce radiation in relatively concentrated, narrow spectral bands when compared to "blackbody" or "graybody" emitters, which typically produce a broad band thermal emission. As a result of the concentrated, narrow spectral band photon emission from such superemitters, thermophotovoltaic (TPV) power systems that are used in conjunction with such superemitters have greater conversion efficiency, i.e., conversion of photon radiation to electricity, than TPV power systems used in conjunction with blackbody emitters operated at the same heat flux. This is due to the typically broad band emissions from such device. However, blackbody emitters can be made to produce a selective spectral band photon emission by using one or more band-pass filters interposed between the emitter and the TPV power system.
Several thermophotovoltaic devices that are used to collect, select, and direct radiation are shown in FIGS. 1-4. Ideally, superemitters emit radiation having a wavelength near, i.e., slightly higher than, the photovoltaic material's band gap. For example, silicon has a band gap at about 1,100 nm, InGaAsP can be tuned to have a band gap covering a wider range, e.g., from 800 nm to about 1700 nm, and ytterbia has a band gap with a peak at 975 nm, i.e., just above the silicon band gap energy. An ytterbia-based mixed oxide emission spectrum is compared with that of holmium oxide in FIG. 5. Multilayered multiband cells made of InGaAsP and other multilayered group III-V compounds can be used to match the ytterbia and holmium based peaks shown.
Although superemitting fiber matrix burners have been developed, they have generally not been found to be effective in very high efficiency/very high fiber temperature device (as described in copending patent applications Ser. Nos. 07/860,777, now U.S. Pat. No. 5,356,487, and 07/695,783, respectively filed on Mar. 27, 1992 and May 6, 1991) when the gas velocity hitting (impinging) the fibers exceeds a certain amount which destroys the fiber.
Superemitter ceramic burners, which emit radiation in a narrow spectrum when heated above their threshold temperature offer the potential for such high-efficiency energy production for a short period of time when the gas pressure and velocity hitting the fibers are very high, i.e., sufficient to cause damage to the fibers over time. However, if the principle of opposing torches is used to provide the thermal energy needed to effect superemitter photon emission, the fibers will see very low gas velocity and will, therefore, provide a high degree of power for long periods of time without damage. Another method to produce a long lived emitter is shown in FIG. 3, wherein the superemitter is coated onto the surface of a transparent light pipe or waveguide.
It is desirable that these superemitter ceramic burners have highly active emissive surfaces. It is also desirable that these superemitters be inexpensive and easy to produce, strong and durable, and have high-temperature and high-energy density capabilities. The intensity of the light emitted from a superemitter increases dramatically with temperature.
The amount of radiant energy emitted and then collected by a photovoltaic cell used in conjunction with such superemitter will also increase dramatically with temperature if most or nearly all the photons can be collected and converted or recycled. Therefore, optical collection systems are desired to produce electric power efficiently, i.e., convert the photon emission from such superemitters to electrical energy. The efficiency of converting photon emission to electric energy increases if the thermal energy of the exhaust gas is recycled by means of a recuperator system, which transfers the thermal energy in the photons and in the exhaust gases to the incoming reactants. The recuperator may increase the temperature of the reactants above the auto-ignition point. To provide for these important energy conservation features, a fuel injection system has been invented that allows the combustion inlet temperature to reach well over auto-ignition.
There is, therefore, a need for an improved photon collection technology for a wide variety of applications such as photon sources for pumping lasers, and providing photons of specific wave bands for such purposes as photolithography, photochemical reactors, etc. and TPV energy generation.